Basically they are different forms of "guessing" the best bitrate for each part of the movie. The problem with guessing is that it makes it almost impossible to keep an accurate average bitrate, so in the end your file may be bigger than you were expecting (and may not fit on the disc). Also, the concept of "constant quality" assumes that a computer is able to judge an image's "quality", which is not exactly true. Some details that may seem irrelevant to a computer are very noticeable to humans, and vice-versa.

The best way to achieve a good distribution of the bitrate with a reliable file size is to use 2-pass VBR. In this mode there is very little guessing involved: the movie is first compressed without any imposed limits on the bitrate, then it's analysed, and finally it's encoded again, keeping the selected bitrate and using the first pass' data to decide how that bitrate should be distributed.

Some programs will do 3- and 4-pass encoding. While this does improve the distribution a bit, the biggest difference is between single-pass (which involves guessing) and 2-pass (which is based on analysis of the footage). Extra passes make small differences.

Generally, if your movie is under 1 hour, you can use CBR @ 8 Mb/s. This will result in the best quality and will take less time to encode. If your movie is too big to use an average of 8 Mb/s, then using 2-pass VBR will ensure that bits are saved in areas that don't really need them, and used in the areas that do (keeping a lower average but letting critical parts go all the way up to 8 Mb/s).